In a step of fixing a toner image which have been transferred on a transferee material of an image forming method via electrophotography, a method of passing between rotating heating device and pressing device is widely popularized for fixing a toner image on a transferee material
In recent years, investigations of energy saving have been conduced in various fields from a view point of preventing global warming, improvement has been progressed such as realization of stand-by power saving employing less energy in an information device such as an image forming apparatus, and on one side, investigations were conducted in lowering fixing temperature in a fixing process which consumes energy most. The fixing temperature is herein means surface temperature set in a heating device surface temperature.
Technologies have been developed for reducing heat capacity of a heating device in a fixing device to shorten warm-up period for this purpose. Concretely, methods are made popular in which thinning a substrate made of aluminum the heating device or, employing a film or a belt for heating device.
While there is an advantage in the fixing device employing the heating device with reduced heat capacity to shorten warm-up period, surface temperature in a region corresponding to a non-image portion in the heating device may rise in excess, or surface temperature in a region corresponding to an image portion in the heating may fall in excess. There are problems particularly that when size or an image pattern of a transferee material is changed after continuous printing out of same image, hot off-set phenomena occurs in a region of a heating device where surface temperature rises in excess, or fixing strength of an image to be formed becomes lower in a region of a heating device where surface temperature falls in excess (see, for example, Patent Document 1).
There is known technology of introducing component having high elastic modulus into a binding resin of a toner for developing electrostatic image to inhibit occurring hot off-set phenomena, generally. However, there is a problem that high glossiness is not obtained since surface of the image formed by introducing high elasticity component is not smooth.
In recent years, a fixing device employing low heat capacity heating device has been developed to a color image forming apparatus, and high glossiness is required for the image as formed (see, for example, Patent Document 2).
It is general to employ a binding resin having so called a sharp melt property in toner for developing electrostatic image for giving the image high glossiness, however there is a problem that anti-hot off-set property cannot be obtained in wide fixing temperature range.
While low temperature fixing is attained by employing a binding resin having softening point low for toner for developing electrostatic image and energy saving is realized, there is a problem to hot off-set phenomena by lowering thermal physical properties of the toner for developing electrostatic image simply.
As described above, it is difficult to dissolve the three problems of high glossiness, low temperature fixing property and anti-hot off-set property in conventional toner for developing electrostatic image simultaneously.